Because the contact is made through a drop of mercury, it is very reliable for small signals.
As long as it is not destroyed by too much current, this is probably the best way to make a contact.

The only thing that I don't like is the magnetic field through the contacts to put the mercury in position to make the contact.
As we all know magnetic materials and fields influence the signal(current).

Dick.

__________________
“Information is not knowledge Knowledge is not wisdom
Wisdom is not truth Truth is not beauty Beauty is not love Love is not music Music is THE BEST”

well , mercury is actually diamagnetic, i e it creates a very small magnetic field by the opposite vector when subjected to an external magnetic field. it is not strong enogh to "move" the mercury unless insanely strong magnetic fields are used (but then the rivet buttons in your jeans tend to follow to...)

therefore mercury wetted relays dont operate by magnetic force to move the
mercury, but rather a magnetic material like iron, which is one of the contacts. the other is the mercury itself.

this is used in some high power relays , since it is quite unsensitive to arcing, but have been succeded by other ways of high power switching.

the typical use for a mercury relay is however a tilt relay , where a drop of mercury is "flowable" inside the body of the relay and depending on position, creates contact or not. it would be a bit awkward to tilt the preamp each time you want to change channel

But, if you look carefully at manufactures like Clare, they have a number of small mercury wetted relays with 0.005 Ohm contact resistance.
I have found quite a number of these relays at electronic dumps.
They can easily be recognised by the arrow pointing in the way they should be mounted.
I have seen them also in the industry on PCB's where small signals switching occurs.

So, my question is still are they suitable for high quality audio regarding the magnetic field needed to make the contact ?

__________________
“Information is not knowledge Knowledge is not wisdom
Wisdom is not truth Truth is not beauty Beauty is not love Love is not music Music is THE BEST”

actually peranders, that depends on which country you live in. perhaps in holland (as I asume is dj_koops homeland) it is still legal?

In sweden all selling,use or discarding of metallic mercury in any form will be banned from 2004 or 2005 i think. This is also true for most mercuric compounds.

by the latest of 2020 all mercury will be collected , transformed into mercuric sulfide (insoluble, ak a mineral cinnober)
and stuffed away in the ground, not unlike nuclear waste. Back to topic...

Anyway, _koop, you are right. I mixed up a couple of different relays. I actually googled a bit and found out that I was (partially) wrong.

As far as I understand it, mercury wetted relays use mercury as a contact material,(probably to improve debouncing) , and also by reducing contact resistance (it is a liquid, filling out all "gaps) but the moving part is still some sort of solid conductor, moved by a magnetic material with a magnetic field, since a magnetic field doesnt affect mercury very much physically.

So I dont think that any magnetic field would influence the signal any worse than a normal relay.

By taking the the environmental issue in consideration , which by me would be the most important technical factor here I would rather use a designated small signal relay

About the environmental issue of mercury here in Holland, I really wouldn't know, and I wouldn't be surprised that it's forbidden to.

Anyway, as said before, I have seen them around.

Added to the excellent reply of Rickard, one other great advantage of mercury wetted relays is their reliability over the years. As long as the coil doesn't burn, it will make a good contact forever if switching signals.

But as these relays are hard to get, I also recommend a small signal relay over every other big relay, if switching signals.

Dick.

__________________
“Information is not knowledge Knowledge is not wisdom
Wisdom is not truth Truth is not beauty Beauty is not love Love is not music Music is THE BEST”